The present invention relates to a mast assembly in a forklift truck having an outer mast supported by a truck body at the front thereof, an inner mast movable upward and downward relative to the outer mast and a lift cylinder for moving the inner mast relative to the outer mast.
Conventionally, various types of forklift trucks are known. Japanese Unexamined Patent Application Publication Nos. 2005-67861 and 8-295496 disclose a forklift truck having a mast assembly 21 a part of which is shown in FIG. 6. The mast assembly 21 has a pair of outer masts 22 (only one being shown in FIG. 6) mounted to the truck body at the front thereof, a mast support 24 mounted to each outer mast 22 at the bottom end thereof and a lower beam 26 connected to the mast support 24. Holes 26a (only one being shown in FIG. 6) are formed in both ends of the lower beam 26. The bottom end of a lift cylinder of the mast assembly 21 is inserted into the hole 26a so that the lift cylinder is supported by the lower beam 26.
There are various types of mast assemblies each having outer and inner masts and a lift cylinder of different sizes for different purpose of use. Thus, the number of components for such different mast assemblies tends to be increased. Furthermore, the lift cylinder is directly fixed to the lower beam which receives the thrust of the lift cylinder in operation to elevate the inner masts. The lower beam is also subjected to twisting force of the outer masts and also the force which is created when the outer masts are opened. The above mast assembly has been disadvantageous in that the above complex forces are applied to the joint which connects the lower beam to the mast support. In the structure wherein the lower beam is connected directly to the outer masts, the joints which connect the lower beam to the outer masts are subjected to the above complex forces. Therefore, there has been the need to increase the strength of the joint.
The supporting structure for the bottom end of the lift cylinder is provided by a lower beam that is a relatively large-sized component. Therefore, this supporting structure needs to be made by machining a large-sized work, which complicates the manufacturing process and increases the manufacturing cost.
The object of the present invention is to provide a mast assembly in a forklift truck, which permits the use of common components, restrains the development of the complex forces and reduces the manufacturing cost.